Why does this throw NullPointerException instead of italicising?

Question

• What I am trying to do is italicise the text inside a JButton, but it throws a Null Pointer Exception.

• This only happens when I click the JButton.

• I have 3 classes, Frame.java, Action.java and Button.java .

• The Null Pointer Exception is thrown in Action.java at the line callTo.setFont(newFont);, on line 27.

• I use JDK11 in Visual Studios Code

Frame.java:

import java.awt.*;
import java.util.HashMap;
import javax.swing.*;

public class Frame {

static JFrame frame = new JFrame();
static JPanel panel = new JPanel();
static BoxLayout layout = new BoxLayout(panel, BoxLayout.Y_AXIS);

static HashMap<String, JButton> buttons = new HashMap<String, JButton>();

static HashMap<JButton, JLabel> italiciseButtons = new HashMap<JButton, JLabel>();
public static void main(String[] args) {
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setMinimumSize(new Dimension(1600, 1000));
    ImageIcon icon = new ImageIcon("C:\\Users\\Patrick\\Downloads\\DragonworldIslandsLogo.png");
    frame.setIconImage(icon.getImage());

    JButton yeet = Button.makeButton("yeet", "Yeet!", Color.GREEN);
    yeet.addActionListener(new Action("italicise", yeet));
    panel.add(yeet);

    frame.add(panel);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}
}

Action.java:

import java.awt.*;
import java.awt.event.*;
import java.util.HashMap;
import javax.swing.*;

public class Action implements ActionListener {
HashMap<JButton, String> buttonCalls = new HashMap<JButton, String>();
JLabel callTo;


public Action(String call, JButton callWith) {
    if (call.equals("italicise")) {
        callTo = Frame.italiciseButtons.get(callWith);
        buttonCalls.put(callWith, call);
    }
}



@Override
public void actionPerformed(ActionEvent event) {
    JButton source = (JButton) event.getSource();
    String called = buttonCalls.get(source);
    if (called.equals("italicise")) {
        Font font = source.getFont();
        Font newFont = font.deriveFont(Font.ITALIC);

        // THE LINE BELOW HAS THE PROBLEM!

        callTo.setFont(newFont);
        callTo.repaint();
    }
}
}

Button.java:

import java.awt.*;
import javax.swing.*;

public class Button {
public static JButton makeButton(String buttonName, String buttonText, Color buttonColour) {
    JButton button = new JButton(buttonText);
    Frame.buttons.put(buttonName, button);
    button.setFont(new Font("Roboto Mono", Font.PLAIN, 18));

    if (buttonColour == Color.BLACK) {
        button.setForeground(Color.WHITE);
    }
    button.setBackground(buttonColour);
    button.setFocusPainted(false);

    return button;
}
}

Thank you ahead of time!